1. Field of the Invention
The invention relates to a driving force control apparatus for a vehicle and a vehicle control method.
2. Description of Related Art
Conventionally, there has been known a driving force control apparatus that is installed in, for example, a four-wheel-drive vehicle, and that is able to variably control distribution of driving force to right and left wheels. Refer to, for example, Japanese Patent Application Publication No. 2009-150484 (JP 2009-150484 A) and Japanese Patent Application Publication No. 5-262156 (JP 5-262156 A).
A driving force control apparatus described in JP 2009-150484 A includes a gear input member connected to a propeller shaft, a gear output member meshed with the gear input member, and a pair of clutch output adjustment mechanisms respectively arranged between the gear output member and right and left rear wheels. By adjusting transmission torque with the use of the clutch output adjustment mechanisms, it is possible to control distribution of driving force to the right and left rear wheels.
A driving force control apparatus described in JP 5-262156 A includes hydraulically-driven right wheel-side clutches and hydraulically-driven left wheel-side clutches, and sets the hydraulic pressures that are supplied to the right wheel-side clutches and the left wheel-side clutches by executing feedback control such that an actual yaw rate approaches a target yaw rate computed on the basis of a steering angle and a vehicle speed.
In a vehicle including the driving force control apparatus described in either one of the documents described above, the turning performance can be improved by transmitting, to the outer wheels during turning, a driving force higher than a driving force that is transmitted to the inner wheels during turning.
However, the actual yaw rate changes as the orientation of the vehicle changes. Therefore, in a control for causing the actual yaw rate to approach the target yaw rate, for example, even when the vehicle understeers and the turning radius thereof deviates from a target turning radius, the actual yaw rate does not coincide with the target yaw rate if the orientation of the vehicle is changed. Therefore, a control for causing the turning radius of the vehicle to approach the target turning radius cannot be executed. As described above, with the control for causing the actual yaw rate to approach the target yaw rate, the turning performance cannot be sufficiently improved in some cases.